The most widely used method for making glass optical fiber involves the use of refractory material induction furnaces to heat a glass preform to fiber drawing temperature. A typical induction furnace for drawing silica fibers is described in R. A. Runk, "A Zirconia Induction Furnace For Drawing Precision Silica Waveguides", Optical Fiber Transmission II Technical Digest (TuB 5-1) (Feb. 22-24, 1977). The induction furnace uses a tubular susceptor of zirconia or other refractory material disposed within a radio frequency coil. After the susceptor is heated to a desired temperature of approximately 2100.degree. C., a glass preform rod is introduced into the middle of the susceptor, known as the hot zone. The preform end is reflowed and a tip portion drops off, attached to a glass strand and permitting the drawing of fiber from the remaining reflowed end. The conventional zirconia draw furnace comprises an outer cylindrical beaker of fused quartz, an inner tube (susceptor) of zirconia and an insulating region therebetween filled with zirconia granules (grog).
Although this method has produced fibers for many years, there has been a persistent problem with particulate contamination of the fiber. Refractory particles can contaminate the fiber as it is drawn, weakening the fiber and causing breakage. The induction furnace, with its refractory susceptor and grog, is a significant source of refractory particles. Moreover facilities where such furnaces are used develop a small but nonetheless significant environmental concentration of such particles.
The standard approach to reducing particulate defects is to draw the fiber under cleanroom conditions. This approach, however, has been less than completely successful. One problem is that cleanroom conditions cannot be established in the critical drawing region where the molten glass is closely spaced with the particulate generating refractory material. Another is that even a minute concentration of refractory particles can present a contamination problem. Accordingly, there is a need for a new method of reducing particle-induced breakage in drawing optical fiber.